changing rears
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Joined: Oct 2008
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From: jefferson,ny
Car: 1986 z28
Engine: 350
Transmission: 700r4
Axle/Gears: ?
changing rears
im changing rears this weekend im going from a 2.73 10 bolt 1 wheel peel to a 9 bolt 3.27 posi with disk brake does anybody know if i'll get better or worse on gas? i own a 92 rs 350 tbi 700r4
Joined: Jul 1999
Posts: 10,950
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From: Orange, SoCal
Car: 1990 Pontiac Trans Am
Engine: 355 TPI siamesed runners
Transmission: Tremec T56
Axle/Gears: 12-Bolt 3.73
Re: changing rears
It depends on your driving style. Remember you'll also need to change the plastic speedo gears in the transmission so your speedometer will read correctly.
Joined: Aug 1999
Posts: 19,282
Likes: 103
From: Lawrence, KS
Car: Met. Silver 85 IROC/Sold
Engine: 350 HO Deluxe (350ci/330hp)
Transmission: T-5 (Non-WC)
Axle/Gears: Limited Slip 3.23's
Re: changing rears
JamesC
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Joined: Jun 2006
Posts: 14
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From: California
Car: 91 camaro z28
Engine: L98 350, Vortech heads, Lt4 cam
Transmission: 700r4
Axle/Gears: 9 bolt 3.27
Re: changing rears
I just swapped to a 9 bolt with 3.27 gears and was wondering do I have to change the portioning valve because it seems to stop fine the the original one.
Joined: Aug 1999
Posts: 19,282
Likes: 103
From: Lawrence, KS
Car: Met. Silver 85 IROC/Sold
Engine: 350 HO Deluxe (350ci/330hp)
Transmission: T-5 (Non-WC)
Axle/Gears: Limited Slip 3.23's
Re: changing rears
If you're swapping from drums to discs, yes for best brake performance. Different braking systems, different proportioning valves.
JamesC
JamesC
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Joined: Aug 2007
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Re: changing rears
My city mileage went up when I went from 2.73's to 3.42's on the rare occasion I could stay out of the gas long enough to calculate the MPG fairly.
It made a world of difference in how quick the car was. It was such a dog with the 2.73's that after the swap it was tough not to get on it a little every stoplight so it changed my driving style. Average MPG went down because the way I drove the car changed but it was well worth it.
The swap to disc from drum in the rear will drop your MPG though if you had the aluminum brake drums, probably a wash with iron drums.
It made a world of difference in how quick the car was. It was such a dog with the 2.73's that after the swap it was tough not to get on it a little every stoplight so it changed my driving style. Average MPG went down because the way I drove the car changed but it was well worth it.
The swap to disc from drum in the rear will drop your MPG though if you had the aluminum brake drums, probably a wash with iron drums.
Joined: Aug 1999
Posts: 19,282
Likes: 103
From: Lawrence, KS
Car: Met. Silver 85 IROC/Sold
Engine: 350 HO Deluxe (350ci/330hp)
Transmission: T-5 (Non-WC)
Axle/Gears: Limited Slip 3.23's
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Re: changing rears
The 10.5" rotors are much heavier than my aluminum drums and a little heavier than the iron drums I had on the shelf that I compared them too as well.
If you are going with 11.75" late TPI PBR caliper or LT1 or 12" LS1 rear rotors the weight difference is substantial.
The extra rotating mass lowers rear wheel HP and MPG. Not by much but a little.
The switch to aluminum drums from iron drums was worth half a tenth on my 12-second Chevelle just from the lower rotating mass and increased power to the rear wheels because of it.
If you are going with 11.75" late TPI PBR caliper or LT1 or 12" LS1 rear rotors the weight difference is substantial.
The extra rotating mass lowers rear wheel HP and MPG. Not by much but a little.
The switch to aluminum drums from iron drums was worth half a tenth on my 12-second Chevelle just from the lower rotating mass and increased power to the rear wheels because of it.
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